Voltage clamp experiments are employed to determine functional and structural characteristics of ionic channels in the squid giant axon. Information concerning these characteristics of the ionic channels is gained by studying the interaction of ions which block the passage of normal charge carriers and by studying the effect of voltage upon the opening and closing ("gating") of channels. Computer simulations are performed of discrete openings and closings of single potassium and sodium ionic channels in nerve and heart using results from probability theory and a random number generator. The gating kinetics of stochastic single K-channels are related to the kinetics of conventionally defined conductances.